I'm thinking about getting an SB-400 to throw in my bag for use as a popup-replacement or hair/rim light (with a PocketWizard).

You mention pocketwizard so I'm assuming you mean off camera and in manual mode?

I had an SB400 only briefly but if I remember correctly there are no controls on the flash. It's meant to be used on the hot shoe and controlled by the camera.

Seeing that you will be using it in manual mode (I'm assuming), there are a tons of used flashes to be had, small one with low profiles, and most with some degree of output control. You could probably buy 20 or more of these pre-digital flashes for what an sb400 costs. I think the sunpack has the same limitations as the sb400.